Photochromic glasses which darken to a pink hue

Abstract: A transparent photochromic glass displaying a pink hue in the darkened state, the glass consisting essentially, expressed in terms of weight percent on the oxide basis, of SiO₂ 52-58 ZrO₂ 1.5-2.8 K₂O 4-8 B₂O₃ 16-21 Li₂O 2-3.5 CaO 2-3.5 Al₂O₃ 6-9 Na₂O 0-2.5 TiO₂ 1.2-2.5 with photochromic elements consisting essentially, as analyzed in weight percent, of Ag 0.15-0.25 Cl 0.25-0.40 Br 0.06-0.13 CuO 0.003-0.007 and containing at least 2 ppm, but less than 10 ppm, Au and up to 0.9% Er₂O₃.

Description: Background of the Invention Transparent photochromic or phototropic glasses are known since U.S. Patent No. 3,208,860, and subsequent patents. An important use is for ophthalmic lenses and non-prescription sunglasses, see U.S. Patent No. 4,190,451 and 4,251,278. The wearer of photochromic eyeglasses not only wishes to have the glass demonstrate good photochromic properties, i.e., the glass quickly darkens to transmissions below about 40% at room temperature, preferably below 30%, and fades quickly, (at least 25 percentage points, preferably at least 30 percentage points, after a five minute withdrawal from the actinic radiation at room temperature), but also wishes to have the glass be cosmetically appealing, especially for sunglasses. Colored photochromic glasses shown by U.S. Patent No. 4,240,836, are heat treated under critically-defined conditions and exhibit orange, red, purple, or blue surface coloration in the undarkened state. Various well-recognized glass colorants have been added to the base photochromic glass composition to impart permanent coloration thereto, see U.S. Patent No. 4,018,965 (rare earth metal oxides Er 2 O 3 , Pr 2 O 3 , Ho 2 O 3 and/or the transition metal oxides CoO, NiO, and Cr 2 O 3 ). U.S. Patent No. 4,251,278 shows photochromic glasses which are essentially colorless in the undarkened state, but which demonstrate a warm brown coloration when exposed to actinic radiation, due to the presence of 1-10 parts per million (ppm) of Pd and/or Au, or transparent photochromic glasses exhibiting a brown coloration in the undarkened state and a deeper warm brown coloration in the darkened state, with more than 10 ppm Pd and/or Au, or transparent, silver halide-containing photochromic glasses containing 1-50 ppm Pd plus up to 1% total of 0-0.5% CoO, 0-1% NiO, and 0-1% Cr 2 O 3 and/or up to 5% total of at least one rare earth metal oxide selected from the group of Er 2 O 3 , Pr 2 O 3 , Ho 2 O 3 , and Nd 2 O 3 , wherein the glass exhibits a particular tint in the undarkened state, complemented with a warm brown hue in the darkened state. U. S. Patent No. 4,746,633 (Mazeau et al.) describes transparent photochromic glasses with additions of up to about 1% total of classic glass colorants such as CeO, Er 2 O 3 , MnO, and NiO and up to about 1% SnO or up to 4 ppm Pd to provide a brown coloration in the darkened state. Studies have indicated a substantial market need for transparent photochromic glasses that, in thicknesses of 2 mm, would darken to a dark range upon exposure to actinic radiation at room temperature, that would fade rapidly when removed from the actinic radiation, and that would exhibit a pink coloration of a definite hue in the darkened state, especially a color as defined by the trichromatic coordinates determined through the trichromatic colorimetric system of the C.I.E. of 1931 utilizing Illuminant C as the light source between the ranges of x >0.3500 and <0.3700 and y >0.3250 and <3450. Summary of the Invention I have discovered that photochromic glasses exhibiting such color can be prepared from glasses having base compositions consisting essentially, expressed in terms of weight percent on the oxide basis, of SiO 2 52-58 Na 2 O 0-2.5 B 2 O 3 16-21 K 2 O 4-8 Al 2 O 3 6-9 CaO 2-3.5 ZrO 2 1.5-2.8 TiO 2 1.2-2.5 Li 2 O 2-3.5 with the photochromic elements consisting essentially, as analyzed in weight percent, of Ag 0.15-0.25 Cl 0.25-0.40 Br 0.06-0.13 CuO 0.003-0.007, wherein at least 2 ppm, but less than 10 ppm, Au is incorporated. Where desired, Er 2 O 3 may be added in amounts up to 0.9% by weight. As will be appreciated, significant additions of Er 2 O 3 give rise to a permanent pink coloration, i.e., the glass in the undarkened state will display a pink tint. Accordingly, where an essentially colorless glass in the undarkened state is desired, the content of Er 2 O 3 will not exceed 0.5%. The concentration of the Er 2 O 3 addition is also governed by the level of Au in the glass. Thus, the content of Au ought not to exceed 9 ppm to insure against a browning of the pink hue. Also, where the Au level is relatively high, the concentration of Er 2 O 3 must be kept high to maintain the color within the prescribed trichromatic coordinates. When present in 2 mm thickness, the inventive glasses in the undarkened state exhibit a luminous transmittance higher than 88%. (Where a very slight pinkish tint is desired or can be tolerated, the undarkened luminous transmittance can be as low as 82% and the darkened color will still fall within the prescribed trichromatic coordinates.) Furthermore, when present in 2 mm thickness, the inventive glasses will demonstrate the following photochromic properties: after an exposure of 15 minutes to actinic radiation at 25°C, the glasses darken to a luminous transmittance between about 20-32%; after an exposure of 15 minutes to actinic radiation at 40°C, the glasses darken to a luminous transmittance between about 35-55%; and after five minutes' removal from the actinic radiation at 25°C will fade back at least 30 and preferably at least 35 percentage points of transmittance such that, where the glass is essentially untinted in the undarkened state, the glass will fade back to a luminous transmittance of at least 55% and preferably at least 60%. Even when the glass is given a permanent pink tint in the undarkened state, the depth thereof will not be so great that the glass will not fade back to a luminous transmittance greater than 50%. As can be recognized from the above description, the basic goal of the present invention was to produce transparent photochromic glasses demonstrating photochromic properties similar to those displayed by the glasses of U. S. Patent No. 4,746,633, supra, that is, photochromic properties within the dark range, but which would exhibit a precisely-defined pink hue in the darkened state, and, if desired, may exhibit a slight pink tint in the undarkened state. I found that glasses demonstrating those properties, coupled with the desired pink tint, could be prepared by carefully controlling the concentrations of the base glass components, including the ranges of the photochromic elements, and then incorporating Au and optionally Er 2 O 3 in strictly restricted amounts. Whereas U. S. Patent No. 4,251,278 supra, discloses the use of Au in amounts of less than 10 ppm, the coloration produced thereby is stated to be a warm brown. Likewise, that patent refers to the use of Er 2 O 3 to impart a permanent pink to the undarkened glass. But, again, a brown coloration is produced in the darkened state. The compositions of the present inventive glasses are outside of those described in that patent. Au is nowhere mentioned in U. S. Patent No. 4,746,633; up to 4 ppm Pd is noted as being useful in providing a brown coloration in the darkened state. Thus, the color demonstrated by the glasses of this subject invention has not been disclosed elsewhere and results from a combination of components not disclosed or suggested in the prior art. Description of the Embodiment Reported below in terms of weight percent on the oxide basis as calculated from the batch is the most preferred base glass composition. Inasmuch as volatilization of those ingredients during melting of the batch is very slight, for all practical purposes the values recorded may be deemed to represent weight percent in the finished glass. The actual batch materials employed can comprise any materials, either oxides or other compounds, which, when melted together, are converted into the desired oxide in the proper proportions. For example, Li 2 CO 3 and CaCO 3 may comprise the sources of Li 2 O and CaO, respectively. The ingredients of the batch were compounded, thoroughly mixed together to assist in obtaining a homogeneous melt, and then progressively transferred into a platinum crucible to be heated through the Joule effect. The batch was melted for 3 hours at about 1400°C. After forming into desired shapes, those shapes were annealed at about 450°C. SiO 2 54.5 ZrO 2 2.6 K 2 O 4.9 B 2 O 3 20.2 Li 2 O 2.8 CaO 2.9 Al 2 O 3 8.6 Na 2 O 1.5 TiO 2 2.0 Table I recites a number of glass compositions illustrating the strict control over composition that must be exercised in order to attain the desired photochromic properties along with a pink coloration is defined by the selected trichromatic coordinates. Thus, the table recites the photochromic elements, as analyzed in weight percent in the finished glass, and the additions of Au, Pd, and Er 2 O 3 in weight percent. Analyses of the photochromic elements have indicated losses during melting of about 30-40% for Cl, 40-50% for Br, 5-15% for Ag, and 0-5% for CuO. 1 2 3 4 5 6 Ag 0.229 0.216 0.216 0.216 0.221 0.241 Cl 0.375 0.380 0.380 0.380 0.358 0.382 Br 0.087 0.083 0.083 0.083 0.080 0.088 CuO 0.004 0.004 0.004 0.004 0.004 0.004 Au 0.0004 0.0003 0.0003 0.0003 0.0005 0.0005 Er 2 O 3 -- 0.3 0.5 0.8 0.3 0.8 7 8 9 10 11 12 Ag 0.189 0.216 0.233 0.218 0.216 0.218 Cl 0.363 0.380 0.369 0.356 0.380 0.369 Br 0.079 0.083 0.080 0.078 0.083 0.080 CuO 0.004 0.004 0.004 0.0035 0.004 0.005 Au 0.0003 0.0003 0.0003 0.0003 0.0003 0.0003 Er 2 O 3 0.3 0.3 0.3 0.3 0.3 0.3 13 14 15 16 17 18 Ag 0.218 0.216 0.220 0.216 0.213 0.226 Cl 0.338 0.380 0.353 0.380 0.364 0.374 Br 0.080 0.083 0.077 0.083 0.115 0.090 CuO 0.004 0.004 0.004 0.004 0.004 0.0065 Au 0.0003 0.0003 0.0003 0.0003 0.0003 -- Er 2 O 3 0.3 0.3 0.3 0.3 0.3 -- 19 20 21 22 23 24 25 Ag 0.227 0.227 0.211 0.238 0.192 0.227 0.227 Cl 0.376 0.366 0.357 0.354 0.353 0.379 0.371 Br 0.088 0.083 0.071 0.071 0.070 0.087 0.085 CuO 0.0065 0.0065 0.004 0.004 0.004 0.004 0.004 Au -- -- -- -- -- -- -- Er 2 O 3 -- -- 1.4 1.4 1.4 -- -- Pd 0.00015 0.0003 0.00015 0.00015 0.00015 -- 0.0001 Samples of each of the above glasses were moved into an electrically-heated furnace and exposed at a temperature of about 650°C for a period of about 15 minutes to develop photochromism therein. In general, temperatures between about 600°-675°C for various lengths of exposure periods have been found to be operable in generating photochromism therein. Subsequently, the samples were withdrawn from the furnace and thereafter ground and polished to a thickness of 2 mm. Table II lists measurements of the photochromic behavior exhibited by the polished samples utilizing the solar simulator apparatus whose principle is discussed in U. S. Patent No. 4,190,451 (Hares et al.). The color of the glasses in the darkened state is defined by the trichromatic coordinates (x, y) determined through the trichromatic colorimetric system of the C.I.E. of 1931 employing Illuminant C as the light source. This colorimetric system and light source are explained by A. C. Hardy in the Handbook of Colorimetry , Technology Press M.I.T., Cambridge, Massachusetts (1936). The color in the darkened glass was determined after an exposure of 20 minutes at 25°C under a source of ultraviolet light ("Black light-blue lamp"). In Table II T o designates the luminous transmittance of a glass in the clear (undarkened) state; T D15 (25) designates the luminous transmittance of a glass in the darkened state after 15 minutes' exposure to the solar simulator source of actinic radiation at 25°C; T D15 (40) designates the luminous transmittance of a glass in the darkened state after 15 minutes' exposure to the solar simulator source of actinic radiation at 40°C; and F5(25) designates the percentage points of luminous transmittance which a glass faded after five minutes' removal from the solar simulator source of actinic radiation at 25°C. 1 2 3 4 5 6 T o 89.8 89.3 87.7 87.3 83.8 82.8 T D15 (25) 26.3 26.2 27.2 27.9 24.7 22.7 T D15 (40) 42.4 44.5 45.9 46.9 38.9 -- F5(25) 34.0 38.9 41.4 40.2 37.7 35.7 x 0.3612 0.3612 0.3581 0.3588 0.3612 0.3585 y 0.3401 0.3391 0.3339 0.3326 0.3329 0.3266 7 8 9 10 11 12 T o 87.3 89.3 87.1 88.0 89.3 88.2 T D15 (25) 28.9 26.2 26.5 28.8 26.2 30.3 T D15 (40) 48.7 44.5 45.2 40.9 44.5 52.2 F5(25) 41.3 38.9 37.5 32.5 38.9 42.1 x 0.3576 0.3613 0.3630 0.3603 0.3618 0.3543 y 0.3367 0.3391 0.3364 0.3339 0.3391 0.3342 13 14 15 16 17 18 T o 88.7 89.3 87.0 89.3 86.9 90.7 T D15 (25) 27.8 26.2 26.6 26.2 24.8 23.7 T D15 (40) 45.8 44.5 45.3 44.5 42.4 42.6 F5(25) 40.8 38.9 38.8 38.9 41.4 43.2 x 0.3613 0.3618 0.3614 0.3618 0.3575 0.3145 y 0.3371 0.3391 0.3368 0.3391 0.3394 0.3156 19 20 21 22 23 24 25 T o -- -- -- -- 87.4 90.0 89.4 T D15 (25) -- -- -- -- 27.2 22.5 25.7 T D15 (40) -- -- -- -- 43.1 33.9 41.4 F5(25) -- -- -- -- 34.5 30.2 34.3 x 0.3364 0.3292 0.3475 0.3431 0.3441 0.3215 0.3485 y 0.3322 0.3233 0.3283 0.3236 0.3279 0.3206 0.3345 Examples 18-25 comprise comparative examples illustrating the careful control that must be exercised over composition in order to achieve the required pink tint. Thus, even apparent very slight changes outside of the designated compositional intervals can place the color outside of the demand trichromatic coordinates and/or leads to a permanent coloration which is too dark. Example 5 is our most preferred composition. Whereas the photochromic properties of Examples 19-22 were not measured, because the base compositions of the glasses are quite similar to those of Examples 23-25, it is believed that the photochromic properties of the former glasses would closely track those of Examples 23-25.

Claims: A transparent photochromic glass displaying a pink hue in the darkened state which, in bodies of 2 mm cross section, will exhibit the following optical properties: (a) a clear (undarkened) luminous transmittance greater than 82%; (b) a darkened luminous transmittance after 15 minutes' exposure to actinic radiation at 25°C between about 20-32%; (c) a darkened luminous transmittance after 15 minutes' exposure to actinic radiation at 40°C between about 35-55%; (d) a rate of fading such that the luminous transmittance after 5 minutes' removal from actinic radiation at 25°C will be at least 30 percentage units higher and will exhibit a luminous transmittance greater than 50%; (e) a x trichromatic coordinate between 0.3500 and 0.3700; and (f) a y trichromatic coordinate between 0.3250 and 0.3450; said glass consisting essentially, expressed in terms of weight percent on the oxide basis, of SiO 2 52-58 ZrO 2 1.5-2.8 K 2 O 4-8 B 2 O 3 16-21 Li 2 O 2-3.5 CaO 2-3.5 Al 2 O 3 6-9 Na 2 O 0-2.5 TiO 2 1.2-2.5 with photochromic elements consisting essentially, as analyzed in weight percent, of Ag 0.15-0.25 Cl 0.25-0.40 Br 0.06-0.13 CuO 0.003-0.007 and containing at least 2 ppm, but less than 10 ppm, Au and up to 0.9% Er 2 O 3 . A transparent photochromic glass according to claim 1 exhibiting a clear luminous transmittance of at least 88% and a rate of fading such that after 5 minutes' removal from actinic radiation the glass will exhibit a luminous transmittance of at least 60%. A transparent photochromic glass according to claim 2 wherein the content of Er 2 O 3 will not exceed 0.5%.

Inventors: GRATEAU LUC, PRASSAS MICHEL, GRATEAU, LUC, PRASSAS, MICHEL
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 1994
Main classification: C03C4
IPC codes: C03C4, C03C3